AC-to-AC converters, such as cycloconverters and matrix converters, are used to transform AC voltage at one frequency into AC voltage at another frequency without the use of an intermediate DC link.
Cycloconverters have been in use since the 1920s, when mercury arc rectifiers were used in large industrial environments in variable-speed motor-drive applications. Generally, these early applications involved converting AC voltage at one frequency to AC voltage at a lower frequency. Recent applications are found in alternative energy systems and uninterruptible power supplies. There is a wealth of literature on the operation and control of these cycloconverters.
In the usual applications, such as industrial motor systems, a substantial inductive load (a motor) smoothes (filters) an output load current into a sinusoidal waveform, with ripple, that tracks a corresponding reference sinusoidal voltage with a phase lag. Commutating the load current between the switches of the cycloconverter has generally presented a significant challenge. The substantially inductive load requires a continuous current path, suggesting a make-before-break switching technique. However, the input voltage source requires that no short-circuit current paths exist, suggesting a break-before-make switching technique. As such, these early cycloconverters could not optimally manage this dichotomy; as a result, they suffered from poor efficiency and extremely poor input power factor and harmonic noise.
Commercial availability of the semiconductor thyristor in the 1960s led to a resurgence of popularity and renewed research on the cycloconverter. The thyristor provided lower power losses compared to mercury-arc rectification devices, and thus allowed for more efficient energy conversion. However, the switching control techniques remained unchanged.
Cycloconverters regained interest in the early 1970s, championed by B. R. Pelly and L. Gungyi. Pelly and Gungyi a produced detailed analysis of the operation of the cycloconverter, as well as of the harmonic analysis. A well-respected publication is B. R. Pelly, Thyristor phase-controlled converters and cycloconverters; operation, control, and performance, New York: Wiley-Interscience, 1971. Commercially available thyristors continued to be improved, resulting in faster switching speeds and lower losses. However, the issue of load current commutation remained problematic. One example of the problematic current commutation that may occur is when the current in a thyristor decays to zero under reverse-bias conditions (freewheeling). Without enough reverse recovery time, a function of the semiconductor physics, this thyristor does not completely turn off and will begin to conduct current as the voltage rises in the forward-bias direction. There are other commutation-failure examples described in the literature.
This load current commutation issue has been practically addressed by applying one of two design methodologies: a circulating current mode or a non-circulating current mode. The circulating current mode involved an overlap of the outgoing and incoming converters. The outgoing converter is comprised of the switch elements that are currently conducting current, and the incoming converter is comprised of the switch elements that will conduct the current after the commutation occurs. In some implementations, both converters are always on. On average, the voltage difference between the converters is made to be zero but instantaneously is non-zero. Inter group reactors (IGR) are used to limit the resulting short circuit current. Instantaneously, reactive power flows from the positive converter (the converter that conducts average positive load current), through the IGR, through the negative converter (the converter that conducts average negative current), and then back into the source. This circulating current ensures that the load current does not became discontinuous and could naturally commutate at exactly the right moment as determined by the characteristics of the load circuit. However, the circulating current mode results in poor efficiency and poor source power factor. These drawbacks, coupled with the difficulty of implementing the phase angle modulation, ensured that the cycloconverter still remains a less common topology.
The non-circulating current mode involves operation of, at most, one converter at a time (positive converter or negative converter). Dead time is added to ensure that the outgoing converter is completely turned off prior to turning on the incoming converter—alleviating the requirement of the inter-group reactor and improving efficiency. However, this dead-time adds distortion at the zero-crossing as the load current becomes zero for a finite time. This distortion also adds to the harmonic pollution of the input sources. Despite these problems, this technique became a very popular implementation of the cycloconverter. A well-known limitation of this technique is the requirement for accurate knowledge of the current zero-cross. Uncertainty in the sensing of the load current due to ripple or other noise can complicate the control action.
Pelly described various commutation methods such as “the fundamental current” and “the first current zero technique.” Theoretically, the fundamental current technique results in commutation at exactly the correct instant. Practically, the fundamental current waveform is difficult to obtain, since low-pass filtering adds phase shift, which is difficult to compensate. Other techniques, such as the first-current zero technique, purposely introduce dead-time by turning off the outgoing converter the instant the actual load current first becomes zero. For a load current with a large ripple component, the outgoing converter is thus turned off for a substantial period of time before the fundamental component of the load current crosses zero. This current commutation issue continues to be a significant hurdle, and commercial cycloconverters function only because of skilful tweaking by talented engineers.
Load current sensing is complicated by detector sensitivity over an entire range of operation from the full load current to the thyristor holding current. To overcome this inherent uncertainly, window detectors have been used with a dead band around the zero current cross. Alternative methods of indirectly sensing load current polarity include sensing the thyristor forward-voltage drop or detecting voltage spikes on diodes as they transition from forward-conducting to reverse-blocking.
Cycloconverters were re-examined in the 1980s and improved by the use of a high-frequency resonant link. The high-frequency resonant link allowed the cycloconverter bridge voltage (input voltage) to be made zero for a period of time. Thus, load current commutation could be performed without danger of shorting the input supply. Most of the papers discussing this method focus on the control and operation of the resonant link.
A circuitry topology referred to as a “matrix converter” has also emerged. The matrix converter has been more broadly used as its applications include both AC—AC conversion and DC-AC conversions. Like the cycloconverter, the matrix converter eliminated a need for internal energy storage and a DC link by connecting the load directly to the source through a matrix of switches. In principle, arbitrary input to output voltage and frequency conversion is possible. However, unlike the cycloconverter, the matrix converter is forced-commutated and does not include freewheeling current paths. The matrix switch configuration needs to provide a current path during operation. As such, simultaneous control and commutation is a significant challenge.
In 1990, a DC-AC conversion method, referred to as a pulse width modulated (PWM) cycloconverter, was introduced. This new conversion method applies AC—AC converter techniques to the challenges of inverters for alternative energy systems and uninterruptible power supplies. There are two techniques in the literature describing the operation of the PWM cycloconverter, both using a square wave derived from a DC source, such as a fuel cell or photovoltaic panel, as the oscillating input voltage source—often called an AC link. In the first technique, the square wave is generated upstream of the cycloconverter by a separate switching power supply. In the second technique, the square wave is generated such that the edge-timing lends itself to a gating process that results in an output waveform at the load that would match the waveform obtained from a conventional pulse width modulated inverter. In either technique, the relatively high frequency of the AC link, dictated by the fidelity requirements of the PWM process, requires proper commutation of load current.
In a switched cycloconverter, commutation can occur by one of two methods, natural or forced. Natural commutation is defined to occur when current commutates solely due to the characteristics of the circuit and without switch action. Hence, the precise timing of the commutation is uncontrolled. Forced commutation occurs when explicit switching is used to purposely commutate current from one switch pair to another or to force a current zero-cross. The non-circulating current method (deadtime method) is an example of forced commutation while the circulating current method (both converters always on plus the IGR) is an example of natural current commutation.
Forced commutation has many well known negative effects such as increased switching losses, generation of electromagnetic interference, and poor waveform quality due to zero-cross distortion and sensitivity to the timing of the switching. The use of the IGR to achieve natural commutation also has substantial losses in the IGR, increased size and weight due to the IGR, and poor input power factor due to the circulating current in the IGR.
In view of the above described techniques, it would be desirable to provide a cycloconverter that facilitates natural commutation and mitigates the necessity for inter-group reactors (circulating current mode) and inter-group switching dead time (non-circulating current mode).